This invention relates to a premixed gas combustion burner, and more particularly to a burner system which is capable of a high degree of modulation and has the high mechanical strength of a metal burner with the thermal stability of a ceramic burner.
Premixed gas burners used in boilers, heat pumps, hot water heaters and other applications provide a high heat release in a small area while providing low pollutant gas combustion product emissions. Generally such premixed gas burners comprise a hollow body having a closed end and an open end into which the premixed gas flows. The burner body includes at least a portion which has a multiplicity of holes through and out which the gas and air mixture from the interior of the body flows. Another member which has the burner flame port perforations, and which in the case of cylindrical burners such as that disclosed in Canadian Patent No. 1,303,958, may be a coaxial shell, or in certain designs may be a substantially planar member known as a deck, is spaced outwardly or downstream from the body of the burner. In the prior art, no effective insulation has been provided between the outlet of the flame port perforations and the combustible mixture within the body.
Premixed gas combustion flames are short with the flame front just beyond or above the burner port or deck surface. Normally the mixture has approximately 30 percent excess air so as to provide cleaner combustion products. At loadings, i.e., heat per unit area, below approximately 6 kilowatts per square decimeter, the burner port surface will be radiant since the velocity of the mixture is low resulting in the flame being positioned on or closely adjacent to the surface. This gives rise to problems of thermal fatigue and high temperature oxidation of the burner port surface or deck, and potential flashback of the flame into the burner body. At higher loadings, e.g. approximately 12 kilowatts per square decimeter and above, the increase in volumetric flow is such that the velocity of the mixture may be increased to the point where the flame front is further from the burner port surface resulting in a blue flame and the surface of the burner ports material is relatively cool so that a burner port surface material comprising stainless steel may be used without insulation. However, even at such higher loadings if the amount of excess air is not or cannot be controlled resulting in inadequate excess air, burner surface overheating may yet result. In certain applications, such as domestic hot water heaters, high loading is desired. In other applications, however, modulation between high and low loadings are desired. The low burner loadings, however, as aforesaid, result in the burner port surface or deck becoming radiant.
Certain of the aforesaid problems have been addressed by the use of a high temperature grade of stainless steel, or more exotic high temperature materials such as Aluchrome, Haynes 230 and other expensive exotic materials. While such materials may withstand high temperature oxidation, and possibly also degradation due to thermal stresses if the assembly permits expansion and is therefore forgiving, the potential for flashback of the combustible mixture is greatly increased since such metals are excellent conductors of heat. Thus, the temperature on the upstream side of the burner port surface may be substantially the same as that on the downstream side, i.e., the temperature beneath a deck may be the same as the temperature on top of the deck. In that case, the temperature of the gas mixture may be raised toward the auto-ignition temperature before the mixture passes through the burner ports.
A known premixed burner devised to overcome these problems utilizes a metal fiber material formed from an alloy of iron, chromium, aluminum and yittrium applied to the burner port surface or burner deck. The metal fiber material must have a port construction identical to that of the deck. Thus, to produce this structure both the metal fiber structure and the deck, which is constructed from stainless steel, must be perforated simultaneously. The metal fiber product provides an insulating feature due to the porosity of the structure. When perforating the metal fiber material simultaneously with the stainless steel deck, the fibers tend to be compressed and the porosity of the structure is reduced. Consequently, the conductivity of the material is increased and the protection is reduced or lost. Problems in producing this system include maintaining the stainless steel deck and the metal fiber perforations aligned, and a tendency to intermittently attach the metal fibers to the burner deck by cold welding in the ports. In operation, the metal fiber structure is prone to erosion due to the slightly acidic nature of the water vapor in the combustion products. Additionally, the maximum safe operating temperature of this material is very close to the actual operating temperatures of the burners in practice, especially when propane gas is used as the fuel. Another difficulty with the use of metal fibers is that it relies on the formation of a protective aluminum oxide coating, and the coating may not properly form or may even break down if the appliance within which the burner is used is operated incorrectly, such as in a reducing atmosphere.